The present invention relates generally to the field of auxiliary equipment for marine seismic cables and, more particularly, to a quick acting collar for the attachment of auxiliary equipment to such a cable, and even more particularly to a collar for the coupling of a depth control device to such a cable.
In the course of conducting marine seismic surveys for oil and gas, it is customary to tow long cables, often 3 to 6 kilometers long, beneath the surface of the water astern of a survey ship. As many as twelve of these cables can be towed at one time. Each cable contains hundreds, and perhaps thousands, of hydrophones as well as the electronic systems required to receive reflected seismic signals and transmit this data back to the survey ship to be recorded. These cables may be constructed using semi-solid materials or oil filled tubes, and they are normally two to three inches in diameter.
When conducting a seismic survey, the cable must be towed at a predetermined depth and means must be provided to maintain this depth at all times. In conventional surveys, the tow depth is usually thirty to fifty feet, plus or minus two feet, which is a very tight tolerance when considering the harsh environment in which such systems typically operate. Maintenance of tow depth is accomplished by the use of depth control devices called xe2x80x9cbirdsxe2x80x9d. A plurality of birds are attached to the cable at intervals of one hundred to three hundred meters. These birds are fitted with horizontally oriented wings that can be commanded to cause the cable to dive or rise depending on the pitch of the wings. The birds are self-powered and receive direction from the survey ship without direct electrical connection to the cable, but by electromagnetic coupling. Other modules are also attached to the cable, and all such modules are referred to herein as auxiliary equipment.
As cables are deployed from their respective reels, the birds must be installed at the required intervals. When the cables are retrieved, the birds must then be removed from the cables before the cables are wound on their storage reels. Deployment and retrieval operations are very critical periods in the overall seismic exploration and these operations must be accomplished with speed and accuracy. At times of sudden changes in sea state for example, this is most important since the cables must be expeditiously retrieved to avoid damage or loss of cables. Even in ideal weather conditions, time is money on seismic survey vessels and the less time that is taken to deploy and retrieve cables the more time can be spent in exploration for hydrocarbons.
It is necessary that the cable be able to rotate freely within the bird attachment locations. This commonly achieved by clamping a split concentric cylinder around the cable which has a concentric groove on the exterior and this assembly functions as the inner part of the bearing. Conventional birds use a concentric outer race, which is attached to the bird and is hinged to mate with the inner bearing and is oriented within a groove on the inner race. This hinged assembly is secured by screws or by a latching device. Such an attaching arrangement may be better understood from Cole, U.S. Pat. No. 3,931,608, incorporated herein by reference.
Each bird has two rotating attachment points allowing the bird to remain suspended beneath the cable at all times. Unfortunately, attachment of the hinged assembly requires a certain amount of dexterity, and in inclement weather, such an operation can lengthen the time required to attach or detach birds from the cable. Thus, there remains a need for a coupling assembly that is simple, easy, and quick to expedite the coupling and decoupling of auxiliary equipment to and from the cables.
The present invention addresses this need in the art by providing a crescent-shaped outer retainer ring adapted to slip easily over a cable and then slide into position over a mating inner race. The outer ring is provided with a spring-loaded cam knob which conveniently conforms to the outer surface of the outer ring and, when the cam knob is turned, it disengages the outer ring from the from the inner race. To attach the outer ring to the inner race, a pin attached to the cam knob rides up on a bevel or slanted plane of the inner race without any further manual action until the pin engages the inner race.